The invention resides in a quasi-hemispherical Fabry-Perot resonator for determining the surface resistance of a thin-film material in the mm wave range.
For the manufacture of information processing components (for example, filters, oscillators) thin-film materials with good conductivity, which may also consist of a high temperature superconductive material (for example, YBCO), are structured in a particular way. The thin material films are formed on large surface area substrates (wafers) by sputter- and vapor deposition processes. For the certification of a suitable film manufacturing process, certain material parameters (among others the transition temperature Te, the critical current density jc) are determined in a non-destructive manner, wherein the surface current resistance Rs is the most important. With localized scanning, which determines this parameter over the wafer surface area of interest in a quantitative manner, the reproductivity and the homogeneity of the superconductive film properties are determined.
Regarding the use of Fabry-Perot resonators for the temperature dependent determination of Rs at low temperatures using superconductive mirror materials (niobium) reports are available by Komiyama et al. (in Appl. Phys. LeH. 64(4), Jan. 22, 1996, p. 562-563 under the title Penetration depth measurements . . . xe2x80x9d) and in the WUB-DIS 94-9 by S. Orbach-Werbig under the title xe2x80x9cOberflxc3xa4chenimpedanz epitaktisch autgewachsener Yba2 Cu3O7-8films at 87 GHzxe2x80x9d. Whereas the first publication reports of no provisions for localized measurements, in accordance with the second publication a displacement was provided for by a hand-operated drive for rotating the probe. Localized Rs determinations at a given temperature (77xc2x0 K.) are performed using normally conductive mirror materials (aluminum): Martens et al. (U.S. Pat. No. 5,239,269).
The convention documents of the MIOP 97 by R. Heidinger and R. Schwab disclose a quasi-hemispherical Fabry-Perot resonator for determining the surface resistance in the millimeter wave range. Changes of the DC conductivity in the mirror materials and changes of Rs resulting therefrom were found over a wide temperature range of 20-340xc2x0 K. They were recorded for various material configurations (Cu, Ag, brass) by way of grade measurements over the whole test specimen, that is, not in a localized manner. In a second resonator with an especially focused mirror configuration, which resonator can be operated only at room temperature, the registration of local Rsxe2x80x94inhomogeneities is demonstrated by way of demonstration mirrors with an abrupt resistance change. The experiment is made in the open, that is, not in a closed environment.
The solution formulas known so far do not provide for any, or at least not for any satisfying, combined localized and temperature dependent quantitative determination of the Rsxe2x80x94values. In the described systems, which are based on superconductive niobium, it is not possible to arrange the coupling geometry in such a way that stray losses at the coupling openings are small. The reason herefor is that, in comparison with the copper used in connection with the invention, niobium is difficult to machine. Another reason is the necessary high-frequency examination of the geometry below the shift temperature of niobium (9.4xc2x0 K.), which does not permit any direct corrective measures. With these experiments, so far, no absolute values for the localized surface resistance Rs have been determined.
Solution formulas of the localized measurements are failing because there is no reliable systematic definition, since the manual object guidance is sluggish and not sufficiently accurate (Orbach (1994)). The localized measurements with (quasi) co-focal Fabry-Perot resonators do not have the necessary stability of the temperature environment because they are not equipped with a kryo-system appropriate for performing T-variable measurements.
It is the object of the present invention to delimit unsuitable film areas by identifying areas with increased surface resistance values, which are the result of outside influences. Predetermined comparison measurements of the temperature depending on Rs over the surface area will provide for a suitable data base for quantifying the amount of the outside influence on the material quality and the local temperature gradient for a systematic correlation with the sputtering or vapor deposition parameters.
In a quasi-hemispherical Fabry-Perot resonator for the non-destructive determination of the surface resistance Rs of electrically conductive thin material films, spherical and planar mirrors are disposed opposite each other in a double shielded cooled resonator space structure supported on individual base plates and the planar mirror, on which a wafer with the thin material film is supported, is mounted on a support arm which extends through the double shield structure. Shield sections through which the support arm extends are supported on pivot arms which are pivotally mounted in the center of the base plates and the shield sections are engaged by the support arm so that they move along with the support arm when the support arm is moved sidewardly for a positioning change of the planar mirror thereby preventing radiation leakage from the resonator space.
The invention resides in the combination of a highly developed invention measuring technique for a high resolution quantitative determination of Rs by means of a (quasi) hemispherical resonator arrangement with copper mirrors and a suitable evaporation kryostatic system which, using two radiation shields and a suitable placement of computer-controlled adjustment elements, provide for a reproducible position and temperature variation of the superconductive thin film sample in the millimeter wave beam.
The inner radiation shield is completely surrounded by an outer shield. Both consist of a base plate through which a coolant flows and on which the mirror cover is disposed. The two base plates are, with respect to the coolant flow, arranged in series wherein the coolant flows first through the inner radiation shield, which is supported on the outer radiation shield in a mechanically stable manner by legs which have a low heat conductivity.
The quasi-hemispherical resonator is supported on the base plate of the inner radiation shield in good heat transfer contact therewith. The spherical mirror is firmly mounted on the base plate in good heat transfer relationship therewith. Symmetrically to the projected resonator axis, the base plate carries at least two heating elements, whose heat input to the spherical mirror is monitored and is controlled by temperature sensors arranged in the heat transfer path to the spherical mirror. The plane mirror is disposed on the movable extension arm, which has a low heat conductivity and therefore is connected to the base plate by way of a flexible (metal) band having a good heat conductivity for maintaining the defined kryosystem temperature. It provides for a certain displacement freedom, particularly a rotation of the mirror up to 360xc2x0 about the resonator axis. Furthermore, the plane mirror can be separately heated by means of a heating element mounted thereon. The temperature of the mirror is monitored and controlled by an associated temperature sensor. The heating element provides for well-defined heat input for obtaining a thermal equilibrium between the mirrors also during warm-up.
The second radiation shield is also supported on the mounting plate in heat transfer uncoupled relationship therefrom. The resonator is mounted on the mounting plate indirectly and the extension arm is mounted with its base directly on the mounting plate.
To accommodate the low-conductivity extension arm, each of the radiation shields includes a window through which the extension arm extends so that they can be removed during necessary manipulations (for example, a change of the sample) within the resonator space. In order to keep the radiation shields always closed such that, during examination, no radiation can escape, a sectionally overlapping double wall is provided in the area of the extension arm. The two wall sections are supported on their respective pivot arms. The support arm for the inner radiation shield is rotatably supported at the center point of the inner base plate and, correspondingly, the support arm for the outer radiation shield is rotatably supported on the support arm of the lower base plate. Both wall sections are movable thereby in a circular motion.
Cryogenic apparatus are often so designed that their cooling capacity can be as small as possible. That means that the masses in the cold cryo-areas should be as small as possible. There is however another opposite need because of a possible thermal de-tuning of the resonator during the selection period of a resonance. This de-tuning should be negligible. Therefore, the two mirrors have a minimum mass or an equally large heat capacity.
Since the heat transfer band provided for a good heat coupling of the plane mirror with the inner base plate must be relatively long to permit complete rotation of the mirror, means must be provided which prevent the heat transfer band to drop into the resonator at any rotational position of the mirror. Therefore the mirror is surrounded by a funnel-shaped collar.
The heat transfer uncoupling between the two base plates is preferably realized by glass fiber legs. The uncoupling between the outer base plate and the mounting plate is achieved in the same way. Other materials, which sufficiently prevent heat transfer and are suitable for the system, may also be used. The support also should prevent the input of mechanical bending moments into the plates. This is particularly true for the support of the mounting plates on the base plate of the vacuum chamber, which extends around the whole arrangement.
The design of the two radiation shields remains the most simple if the adjustments on the resonator during measurements are limited to the extension arm. Therefore, all adjustment elements for the extension arm are preferably disposed outside the two radiation shields. Furthermore, they are arranged outside the radiation shields in order to prevent the input of parasitic heat into the resonator space. Is also important for the operation of the adjustment elements that they are disposed in a space with room temperature. Only then can they be operated properly.
With the resonator according to the invention, a method for the non-destructive and contact-free determination of the localized and temperature-dependent surface resistance Rs of electrically conductive thin-film materials is made possible.
Both surface-coated mirrors are adjusted with respect to each other. The plane mirror is mounted on the extension arm, which is mounted so as to be movable with three degrees of freedom. In this way, each point of the exposed surface of the thin film deposited on the planar mirror can be moved into the focus of microwave emitted from the uncoupling opening of the spherical mirror in a delicately stepped manner.
By way of the two radiation shields, defined cryogenic environments are provided which permit quantification of the temperature influence.
The measuring process is performed under the control of a computer in order to characterize a measured thin film layer within acceptable, particularly economically acceptable periods.
The microwave reflected from the test sample is recorded by a sensitive detector system whose input is formed by the uncoupling opening of the spherical mirror directly adjacent the uncoupling opening. The transmission of the microwave is recorded as a function of the frequency and is parameterized in the computer by the adaptation according to the smallest error squares following a Lorentz-distribution. From this, the grade value is determined, which is corrected by the contributions of the reflection losses at the spherical mirror (Surface resistance Rsph) and the dispersion losses at the in-coupling and out-coupling openings. Finally, the location and temperature-dependent value of the surface resistance is determined therefrom.
The location- and temperature variable measurement points, which are obtained by a fully computer-controlled measurement process control, permit, for the first time, a quantitative qualification of the various film areas in the form of a model-based adaptation and a temperature gradient determination. In this way, a use-relevant data basis is provided for the certification and a well-directed improvement of the film manufacture.